If the page has a scrollbar you will be able to scroll it if the RadWindow is modal. This is due to the fact that you cannot take an element from the page on top of the scrollbar (i.e. the modal overlay). If you wish to disable the scrollbars you can do so when the RadWIndow is shown and restore them when it is closed, for example:
//store the overflow
bodyOverflow = document.body.style.overflow;
htmlOverflow = document.documentElement.style.overflow;
//hide the overflow
oWnd = window.radopen(
//restore the overflow
document.body.style.overflow = bodyOverflow;
document.documentElement.style.overflow = htmlOverflow;
If I have misunderstood your issue and you are actually getting scrollbars because you show it then what you can do is try to set its dimensions in the markup, so that it will automatically calculate its position according to them, without the added moving and resizing.
You can also try to call the moveTo and resize functions with a small timeout (you can start with about 300 for example and reduce it if it works), and even 0 milliseconds may suffice.
the Telerik team